The following description relates to electronic signatures. An electronic signature can be used to authenticate the contents of an electronic document by the signer, similar to a manual signature handwritten in ink. Electronic signatures are also used when electronically transmitting documents or messages, to identify the sender and authenticate the message data.
Conventional electronic signatures use public-key encryption. Public key cryptography uses public-private key pairs for creating an electronic signature, electronic signature verification and encryption and decryption of data for security during electronic transmission. A public key owned by an individual receiving the data (the “recipient”) is used by a sender to encrypt the data. The recipient then uses the recipient's corresponding private key to decrypt the data. In order to encrypt the data, the sender must have access to the recipient's public key. Alternatively, a private key owned by the sender can be used to encrypt the data, and the recipient then uses the sender's public key to decrypt the data, and therefore requires access to the sender's public key.
When electronically signing data, a sender signs the data using the sender's private key. This operation can involve using the private key to encrypt a “cryptographic hash” of the data that is being signed, and then making available to the recipient the signed data and the encrypted hash. The recipient verifies the signature by computing a hash over the data using the sender's public key, decrypting the encrypted hash of the signature and comparing the two hashes. If the hashes match, then the data is authenticated.
Typically, a public key for another individual (the sender, for example) is obtained by obtaining an individual's public key certificate directly or indirectly from that individual. A certificate is an electronic data object including a public key, and can be issued by a trusted third party, such as a certificate authority, that verifies the identity of the certificate holder. The certificate can also include the name of the certificate authority and the name of the individual or entity for whom the certificate is issued. The recipient of an electronically signed document can verify the identity of the sender (signer) by verifying the certificate of the sender. A public key certificate for someone else can be obtained in a number of ways, for instance, the certificate can be found in a searchable database on a server, or the certificate owner can include the certificate as an attachment to an e-mail message sent to a recipient. A corresponding private key certificate must be kept secret; for example, the certificate can be protected by a password or other organizational measures.